In the usual process for the manufacture of lacquered wires, the wire is passed through a solution of lacquer, the excess solution picked up in the bath is removed with suitable stripper systems and the wire is then passed through a furnace shaft. The energy supplied in the furnace is first used to evaporate the lacquer solvent and then to effect cross-linking which results in film formation. To obtain lacquer layers of the usual thicknesses, this process must be repeated about 6 to 8 times. In the lacquers employed, the solvent usually amounts to more than 50% of the lacquer and, in the worst cases, to even more than 80%. This ballast, which does not take part in formation of the film, does of course have numerous disadvantages: Unnecessarily large volume, risk of fire or explosion in transport and storage, additional energy requirement for evaporation of the solvent from the lacquer film, considerable environmental pollution and considerable cost and effort to reduce the pollution to the permissible level.
Isocyanate based stoving lacquers which contain solvent and are stable in their viscosity can only be obtained from blocked isocyanates. By blocked isocyanates are meant compounds in which the isocyanate groups are inactivated by reaction with Zerewitinoff active compounds such as malonic esters, ketoximes, phenols, lactams and CH-acidic compounds. Systems of this kind have been described, for example, in German Offenlegungsschrift No. 1,644,794.
This inactivation must be maintained not only at normal temperature but also at the elevated temperatures occurring in storage and transport in order to prevent any change in viscosity or even gelling.
On the other hand, the inactivation must not be too stable because otherwise the desired cross-linking reaction with the component which contains isocyanate active groups will not occur at all in the stoving process or only too slowly to allow for any acceptable production speeds.
On account of these limitations, the choice of blocking agents which can be used for stabilizing the isocyanates used in practice nowadays is restricted to only a few substances, e.g. phenols, ketoximes or caprolactum.
If attempts are made to reduce the proportion of solvent in such lacquers insofar as the solubility of the lacquer allows this, processing difficulties arise because the solids content, viscosity, obtainable thickness of film and levelling properties correlate with each other, depending on the stripper systems employed. The possibility of obtaining a more favorable viscosity, i.e. a lower viscosity, by increasing the temperature is strictly limited because, above a certain temperature the blocking effect of the isocyanate is eliminated so that the viscosity again rises, followed by gelling.
This invention provides new coating substances which have a reliable stability at elevated temperatures, are low viscosity melts at these temperatures, can be applied to the substrate in the form of these melts and can subsequently be reactivated by increasing the temperature and thus cross-linked to films of excellent quality. Numerous advances in processing techniques are thus combined in these substances. In systems which contain solvent, there is always the risk that premature hardening of the surface will form a barrier layer for the solvents still contained in the interior. If these solvents escape due to the high vapor pressure under the conditions of stoving, craters or other faults in the lacquer layer usually occur. This risk must be countered by a special temperature distribution in the stoving shaft. These systems are therefore particularly sensitive to fluctuations in temperature and, therefore, it was not possible to increase the reaction velocity by increasing the temperature. The possibility of increasing the size of the molecule in the stoving process was therefore slight and it was therefore generally necessary to use a product which was already substantially prefinished with regard to its molecular weight.
Since this difficulty does not arise in solvent-free processes, in other words since more heat can be applied to the system without any deleterious effect, a higher reaction velocity can be obtained and consequently products with a lower molecular weight can be used. This in turn makes it possible to produce coating substances with such low viscosities that they can even be used to coat copper wire which has a diameter of less than 0.1 mm.
Another particular advantage of using solvent-free coating substances, apart from that of avoiding the difficulties of solvent recovery, is that thicker layers can be obtained with each individual application since, with the possible exception of the blocking agent, practically all the material applied to the substrate forms part of the film. The final thickness required can therefore be obtained with only a few applications and, in the ideal case, with only one application. At least in the case of wire lacquering it now becomes possible to lacquer a plurality of wires simultaneously, using an already existing lacquering plant. This opens up new possibilities for the construction of the lacquering furnace, for example, each wire can be passed through a tube which is heated by induction or, especially in the case of conductors of large cross-section, the material which is to be applied as coating may itself be heated to the required stoving temperature by high frequency heating means.